Method for producing ceramic parts

ABSTRACT

The present invention relates to a method for manufacture of ceramic parts used for protecting semiconductor devices such as IC, LSI, etc. against anomalous high voltages such as noise, pulse, static electricity, etc. and the object of the present invention is to provide ceramic parts capable of removing or restraining high-frequency noises on signal lines by reducing the impedance of the devices. 
     In order to attain the above object, according to the present invention, a pulse voltage of 50 kV in maximum value which requires a time of 200 nanoseconds or less for reaching the maximum value from an initial value and a time of 1 microsecond or less for returning to the initial value through the maximum value and which has an energy of 0.5 joule or lower is applied at least once between the electrodes (2).

TECHNICAL FIELD

The present invention relates to a method for producing ceramic partsused for protecting semiconductor devices such as IC and LSI used inelectronic equipments and electric equipments against high-frequencynoises and anomalous high voltages such as noise, pulse and staticelectricity.

BACKGROUND ART

Recently, semiconductor devices such as IC and LSI have been widely usedin electronic equipments or electric equipments for miniaturization ofthe devices and for giving multifunctions to the devices. However, theuse of the semiconductor devices causes decrease in resistance of theelectronic equipments and electric equipments against anomalous highvoltage such as noise, pulse or static electricity.

In order to ensure the resistance of electronic equipments and electricequipments against anomalous high voltage such as noise, pulse or staticelectricity, film capacitors, electrolytic capacitors, semiconductorceramic capacitors, laminated ceramic capacitors are used. These haveexcellent characteristics for absorption and control of noises ofrelatively low voltage or high-frequency noises, but they exhibit noeffect on pulses or static electricity of high voltage and sometimescause wrong operation or rupture of semiconductor devices.

In order to absorb and control pulses or static electricity of highvoltage, SiC and ZnO type varistors are used. However, these have noeffect for absorption and control of noises of relatively low voltageand high-frequency noises and are apt to bring about wrong operation.

Under the circumstances, SrTiO₃ type varistors have been developed to beeffective against noises of relatively low voltage or high-frequencynoises and pulses or static electricity of high voltage.

However, owing to the high impedance of elements, the SrTiO₃ typevaristors also have the problems that they are low in the effects toabsorb and control pulses of relatively low voltage or high-frequencynoises applied to signal lines.

The inventors have noticed that the high impedance is caused by abarrier formed at electrode part. Accordingly, the object of the presentinvention is to attain the high effects to absorb and control pulses ofrelatively low voltage and high-frequency noises applied to signal linesby reducing the impedance by destroying the barrier formed at theelectrode part.

DISCLOSURE OF THE INVENTION

In order to attain the above object, the present invention provides amethod for manufacturing a ceramic part according to which a pulsevoltage having a maximum value of 50 kV and an energy of 0.5 joule orlower and requiring a time of 200 nanoseconds or less for reaching themaximum value from an initial value and a time of 1 microsecond or lessfor returning to the initial value through the maximum value is appliedonce or more times between electrodes.

According to the above method, the following effects are obtained. Thatis, when an electrode layer is formed on a ceramic element comprising acomposition mainly composed of, for example, SrTiO₃ and fired in areducing atmosphere and then the ceramic layer and the electrode layerare simultaneously fired in a neutral or oxidizing atmosphere, thesurface of the electrode layer is oxidized to produce a portion of highresistance and a barrier is formed on the surface and inside of theelectrode layer to cause increase in impedance of the element andincrease in tan δ together with decrease in electric capacity and as aresult, the effects to absorb and control noises are damaged. Thisbarrier is generated by oxidation of the surface and inside of theelectrode and since the height of the barrier is relatively low, thebarrier can be removed by application of a relatively small energy suchas electric energy without causing variation in varistor voltage ordeterioration of the non-linear coefficient of the voltage of partshaving other characteristics such as varistor characteristics.

As shown in the present invention, by applying as an electric energy asteep wave pulse voltage which is short in time required for reachingthe maximum value and in time required for returning to the initialvalue, the energy is concentrated into the portion weakest in thebarrier formed at the interface of the electrode layer and breaksthrough a part of the barrier whereby resistance of the barrier can bereduced. When the electric energy applied is too large, the barrierformed at grain boundary of the ceramic layer is damaged and functionspossessed by the parts, especially capacitor function is deteriorated.Accordingly, by applying a steep wave pulse voltage of relatively smallenergy, only the barrier formed at the interface of the electrode layercan be removed and the impedance inherently possessed by the parts canbe taken out without causing deterioration of capacitor characteristicsof the parts and without giving influence to the varistorcharacteristics. Thus, noise attenuation factor can be increased withdecrease in impedance and the high frequency noise entering into signallines can be absorbed and controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart which shows the production steps in Example 1 ofthe present invention.

FIG. 2 is a cross-sectional view of the ceramic part produced in Example1.

FIG. 3 is a cross-sectional view of the ceramic part produced in Example2.

FIG. 4 is a cross-sectional view of the ceramic part produced in Example3.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be illustrated by the following examples.

EXAMPLE 1

99.2 mol % of SrCO₃, CaCO₃ and TiO₂ so as to obtain the compositionalratio (Sr₀.98 Ca₀.02)₀.995 TiO₃ as a first component, 0.3 mol % of Nb₂O₅ as a second component, 0.2 mol % of MnCO₃ and 0.1 mol % of Cr₂ O₃ asa third component and 0.2 mol % of SiO₂ as a fourth component wereweighed, mixed and ground in a ball mill for 20 Hr, calcined at 800° C.for 2 Hr in dry air, and ground again for 20 Hr in a ball mill to obtaina powder of 2.0 μm or smaller in average particle size. To the thusobtained powder was added 10 wt % of an organic binder such as polyvinylalcohol and the powder was granulated. A disc-like ceramic element 1 of10 mm in diameter×1 mm thick as shown in FIG. 2 was prepared therefrom.

Thereafter, this ceramic element 1 was calcined at 1000° C. for 1 Hr.Then, an electrode paste comprising Pd or the like was coated on thefront and back sides of this ceramic element 1 by screen printing or thelike and dried at 120° C. The coated ceramic element was fired, forexample, at 1410° C. for 5 Hr in a reducing atmosphere of N₂ :H₂ =9:1and then, reoxidized at 1080° C. for 2 Hr in the air to form electrodes2 on the front and back sides of the ceramic element 1. Then, lead wire3 was attached onto the respective electrodes 2 by an electroconductiveadhesive 4 such as a solder and these were externally coated with resin5 such as epoxy resin. A steep wave pulse as shown in Table 1 wasapplied between the both electrodes 2 of the resulting ceramic partthrough lead wire 3. Changes in characteristics before and afterapplication of the pulse are also shown in Table 1.

The steps are shown in FIG. 1.

                                      TABLE 1                                     __________________________________________________________________________    Steepness            The number          After application                    of wave Wave                                                                              Peak     of appli-           of pulse                                crest                                                                              tail                                                                              voltage                                                                            Energy                                                                            cation of                                                                            ΔC                                                                          ΔV.sub.0.1mA                                                                 Δα                                                                    tanδ                                                                        ESR                              No.                                                                              (nsec)                                                                             (μsec)                                                                         (kV) (J) pulse  (%) (%)  (%) (%) (mΩ)                       __________________________________________________________________________    1* --   --  --   --  --     --  --   --  10.0                                                                              2000                             2  2    0.1 20   0.10                                                                              10     +7.2                                                                              -0.3 -0.1                                                                              3.2 75                               3  10   0.1 20   0.10                                                                              10     +6.3                                                                              -0.3 -0.1                                                                              4.2 105                              4  80   0.1 20   0.11                                                                              10     +5.5                                                                              -0.3 -0.1                                                                              4.5 110                              5  170  0.1 20   0.11                                                                              10     +2.8                                                                              -0.4 -0.1                                                                              4.9 112                              6**                                                                              250  0.1 20   0.12                                                                              10     +1.4                                                                              -0.4 -0.1                                                                              8.8 280                              7  5    0.5 25   0.15                                                                              1      +7.9                                                                              -0.3 -0.1                                                                              3.0 65                               8  5    0.8 25   0.16                                                                              1      +8.2                                                                              -0.3 -0.1                                                                              2.8 60                               9**                                                                              5    1.5 25   0.17                                                                              1      +9.7                                                                              -0.9 +0.2                                                                              8.5 1200                             10 5    0.1 5    0.01                                                                              5      +7.2                                                                              -0.2 -0.1                                                                              3.4 177                              11 5    0.1 10   0.02                                                                              5      +7.9                                                                              -0.2 -0.1                                                                              3.0 110                              12 5    0.1 30   0.22                                                                              5      +8.6                                                                              -0.3 -0.1                                                                              2.8 70                               13 5    0.1 40   0.40                                                                              5      +9.5                                                                              -2.5 +0.2                                                                              3.5 42                               14**                                                                             3    0.1 55   0.45                                                                              5      +15.9                                                                             -7.4 +11.0                                                                             18.9                                                                              55                               15 2    0.1 25   0.20                                                                              10     +7.5                                                                              -0.2 -0.1                                                                              3.7 60                               16 2    0.1 25   0.40                                                                              10     +7.9                                                                              -0.5 -0.1                                                                              3.5 57                               17**                                                                             2    0.1 25   0.76                                                                              10     +28.4                                                                             +15.3                                                                              +15.8                                                                             26.4                                                                              1750                             18 2    0.1 25   0.15                                                                              100    +7.2                                                                              -0.1 -0.1                                                                              4.2 70                               19 2    0.1 25   0.15                                                                              1000   +7.2                                                                              -0.1 -0.1                                                                              4.2 70                               20 2    0.1 25   0.15                                                                              10000  +7.2                                                                              -0.1 -0.1                                                                              4.2 68                               21 2    0.  25   0.15                                                                              50000  +7.3                                                                              -0.1 -0.1                                                                              4.2 72                               __________________________________________________________________________     (1) The mark * shows application of no pulse.                                 (2) The mark ** shows comparative examples.                                   (3) The steepness of wave crest means a time required for the pulse           reaching the maximum value from the initial value.                            (4) The wave tail means a time required for the pulse returning to the        initial value through the maximum value.                                      (5) The peak voltage means the maximum value of the pulse.                    (6) The energy means the energy of one pulse.                                 (7) ΔC shows the rate of change in electric capacity before and         after application of the pulse.                                               (8) ΔV.sub.0.1mA shows the rate of change in varistor voltage befor     and after application of the pulse.                                           (9) Δα shows the rate of change in nonlinear index before and     after application of the pulse.                                               (10) ESR means equivalent serial resistance in resonance frequency.      

EXAMPLE 2

99.2 mol % of SrCO₃, CaCO₃ and TiO₂ so as to obtain the compositionalratio (Sr₀.85 Ca₀.15)₀.985 TiO₃ as a first component, 0.3 mol % of Nb₂O₅ as a second component, 0.2 mol % of MnCO₃ and 0.1 mol % of Cr₂ O₃ asa third component and 0.2 mol % of SiO₂ as a fourth component wereweighed, mixed and ground in a ball mill for 20 Hr, calcined at 800° C.for 4 Hr in dry air, and again mixed and ground for 80 Hr in a ball millto obtain a powder of 1.0 μm or smaller in average particle size. Thethus obtained powder was mixed with an organic binder such as butyralresin and an organic solvent to prepare a slurry. The slurry was moldedby a sheet-molding method such as doctor blade method to obtain a greensheet 6 having a thickness of about 50 μ m as shown in FIG. 3.

A given number of the green sheets 6 were stacked to form an undermostunavailable layer. An inner electrode 7 comprising Pd or the like wasprinted on said layer by screen printing or the like and dried. Thereonwere further stacked a given number of the green sheets 6 and then, theinner electrode comprising Pd or the like was again printed by screenprinting or the like and dried. In this case, the inner electrodes 7were printed so that they are distributed alternatively one by one tothe opposite (different) directions in the laminates. In this way, agiven number of the green sheets 6 and a given number of the innerelectrodes 7 were stacked and finally, a given number of the greensheets 6 were stacked to form an uppermost unavailable layer. These werepressed and bonded under heating to laminate them. The laminate was cutto a given shape to form a laminate 8 shown in FIG. 3.

Then, this laminate 8 was calcined and degreased at 800° C. for 40 Hr inthe air, then fired at 1310° C. for 5 Hr in a reducing atmosphere of,for example, N₂ :H₂ =9:1, and thereafter reoxidized at 980° C. for 2 Hrin the air. Thereafter, an electrode paste comprising Ag or the like wascoated on the edge faces of the inner electrodes 7 exposed at thedifferent edges and fired at 700° C. for 10 minutes in the air to formouter electrodes 9. A steep wave pulse as shown in Table 2 was appliedbetween the outer electrodes 9 of the thus obtained ceramic part.Changes in characteristics before and after application of the pulse arealso shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Steepness             Before appli-                                                                         After appli-                                                                             After                                of wave  Wave                                                                              Peak     cation of pusle                                                                       cation of pulse                                                                          application                          crest    tail                                                                              voltage                                                                            Energy                                                                            V.sub.0.1mA (V)                                                                       V.sub.0.1mA (V)                                                                       ΔC                                                                         tanδ                                                                        ESR                              No.                                                                              (nsec)                                                                              (μsec)                                                                         (kV) (J) X   σ                                                                           X   σ                                                                           (%)                                                                              (%) (mΩ)                       __________________________________________________________________________     1*                                                                              --    --  --   --  12.5                                                                              2.25                                                                              --  --  -- 25.8                                                                              1850                             2  5     0.1 25   0.20                                                                              6.0 2.45                                                                              6.0 2.46                                                                              +7.5                                                                             10.1                                                                              90                               3  5     0.1 25   0.20                                                                              8.2 1.88                                                                              8.2 1.87                                                                              +7.9                                                                             8.5 75                               4  3     0.1 25   0.20                                                                              9.5 1.71                                                                              9.5 1.71                                                                              +6.3                                                                             7.6 70                               5  3     0.1 25   0.20                                                                              10.7                                                                              1.86                                                                              10.7                                                                              1.85                                                                              +6.5                                                                             6.8 68                               6  3     0.1 25   0.20                                                                              12.3                                                                              1.82                                                                              12.3                                                                              1.83                                                                              +7.0                                                                             6.5 55                               7  2     0.1 30   0.20                                                                              14.8                                                                              2.12                                                                              14.7                                                                              2.13                                                                              +7.4                                                                             5.0 50                               8  2     0.1 30   0.20                                                                              16.2                                                                              2.34                                                                              16.1                                                                              2.35                                                                              +7.6                                                                             4.8 72                               9  2     0.1 30   0.20                                                                              20.7                                                                              3.21                                                                              20.6                                                                              3.28                                                                              +7.8                                                                             4.3 85                               __________________________________________________________________________     The mark * shows application of no pulse.                                     The pulse was applied ten times.                                         

EXAMPLE 3

Green sheet 6 was obtained in the same manner as in Example 2. A givennumber of the green sheets 6 were stacked to form an unavailable layerof an undermost layer. An inner electrode 10 comprising NiO or the likewas printed thereon by screen printing or the like and dried. Thereonwere further stacked a given number of the green sheets 6 and an innerelectrode 10 comprising NiO or the like was again printed by screenprinting or the like and dried. In this case, the inner electrodes 7were printed so that they are distributed alternatively one by one tothe opposite (different) directions in the laminates. In this way, agiven number of the green sheets 6 and a given number of the innerelectrodes 10 were stacked and finally an uppermost unavailable layerwas formed by stacking a given number of the green sheets 6. These werepressed and bonded under heating to laminate them. Then, the laminatewas cut to a given shape to form a laminate 11 shown in FIG. 4.

Then, the laminate 11 was calcined and degreased at 900° C. for 20 Hr inthe air. Thereafter, an electrode paste comprising NiO or the like wascoated on edge faces of the inner electrodes 10 exposed at the differentedges and fired at 1210° C. for 10 Hr in a reducing atmosphere of, forexample, N₂ :H₂ =9:1 to simultaneously carry out reduction of thelaminate 11 and reduction of the outer electrode 12 comprising NiO orthe like.

Thereafter, an electrode paste comprising Ag or the like was coated onthe surface of the outer electrode 12 and reoxidized at 900° C. for 3 Hrin the air to form an outer electrode 9. Then, onto the outer electrode9 are applied Ni plating, for example, by electrolytic process and thensolder plating (not shown). A steep wave pulse as shown in Table 3 isapplied between the outer electrodes 9 of the thus obtained ceramicpart. Changes in characteristics before and after application of thepulse are also shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Steepness             Before appli-                                                                         After appli-                                                                              After                               of wave  Wave                                                                              Peak     cation of pusle                                                                       cation of pulse                                                                           application                         crest    tail                                                                              voltage                                                                            Energy                                                                            V.sub.0.1mA (V)                                                                       V.sub.0.1mA (V)                                                                       ΔC                                                                          tanδ                                                                        ESR                             No.                                                                              (nsec)                                                                              (μsec)                                                                         (kV) (J) X   σ                                                                           X   σ                                                                           (%) (%) (mΩ)                      __________________________________________________________________________    1* --    --  --   --  12.5                                                                              2.19                                                                              --  --  --  28.9                                                                              6500                            2  2     0.1 25   0.20                                                                              5.5 2.52                                                                              5.5 2.53                                                                              +6.5                                                                              12.3                                                                              50                              3  2     0.1 25   0.20                                                                              7.4 1.93                                                                              7.4 1.93                                                                              +7.2                                                                              10.1                                                                              55                              4  3     0.2 25   0.20                                                                              9.5 1.84                                                                              9.5 1.83                                                                              +7.0                                                                              10.2                                                                              54                              5  3     0.2 25   0.20                                                                              11.0                                                                              1.76                                                                              11.0                                                                              1.75                                                                              +6.4                                                                              9.9 52                              6  5     0.2 25   0.20                                                                              13.6                                                                              1.80                                                                              13.5                                                                              1.80                                                                              +6.2                                                                              9.2 60                              7  5     0.2 25   0.20                                                                              18.9                                                                              2.14                                                                              18.8                                                                              2.16                                                                              +6.0                                                                              8.6 75                              8**                                                                              2     0.1 25   0.45                                                                              25.4                                                                              2.36                                                                              21.7                                                                              2.52                                                                              +30.4                                                                             35.2                                                                              1980                            __________________________________________________________________________     The mark * shows application of no pulse.                                     The mark ** shows a comparative example.                                      The pulse was applied ten times.                                         

In the above Examples 1-3, the compositions of the ceramic powder shownare only a part of the combinations of some components with the maincomponent having the basic composition of SrTiO₃ shown by ABO₃ where apart of Sr is replaced with at least one element of Ba, Ca and Mg.However, any compositions may be used as far as they contain SrTiO₃ as amain component and have both the functions of a capacitor and avaristor. Furthermore, in this case, the ratio A/B is 0.95≦(A/B)≦1.05.This is because if the ratio is less than 0.95 a second phase mainlycomposed of Ti is formed and varistor characteristics are deterioratedand if it is more than 1.05 the dielectric constant decreases and thecapacitor characteristics are deteriorated. Moreover, the ceramic powdermay be ZnO or TiO₂. The electrode 2, the inner electrodes 7, 10, and theouter electrodes 9, 12 may be formed of at least one of Au, Pt, Rh, Pd,Ni, Cr, Zn and Cu or alloys thereof. The unavailable layer and theavailable layer in FIGS. 3 and 4 were formed by stacking thin greensheets 6, but may comprise one thick green sheet. As can be seen fromTable 1, Table 2 and Table 3, the capacitor characteristics such aselectric capacity, tan δ and ESR changed greatly by the application ofpulse while the varistor characteristics showed substantially no changein both the absolute value and the standard deviation. Furthermore, inthe present invention the wave of the steep pulse applied is specifiedso that the time required to reach the maximum voltage is 200nanoseconds or less because the higher rising steepness (steepness ofcrest) of pulse voltage is effective to break the oxidized layer on thesurface and inside of the electrode and when the steepness exceeds 200nanoseconds the effect considerably decreases. Moreover, the timerequired for returning to the initial value (wave tail) is specified tobe 1 microsecond or less because the longer wave tail is prone to affectthe characteristics, especially the capacitor characteristics and whenthe wave tail exceeds 1 microsecond, the electric capacity sharplydecreases.

The maximum value of the voltage to be applied is specified to be 50 kVor lower because with increase of the voltage applied the deteriorationof the barrier formed at grain boundary of the ceramic element becomesserious and when the applied voltage exceeds 50 kV, both the capacitorcharacteristics and varistor characteristics are deteriorated. Moreover,the energy of pulse applied is specified to be 0.5 joule or less becausewhen it exceeds 0.5 joule, the device generates much heat to causedeterioration of both the capacitor characteristics and varistorcharacteristics. The preferable conditions for application of the pulseare as follows: the time required for reaching the maximum voltage: 50nanoseconds or less; the time required for returning to the initialvoltage: 500 nanoseconds or less; the voltage applied: 3-25 kV; and theenergy: 0.3 joule or less.

INDUSTRIAL APPLICABILITY

As explained above, according to the present invention, a steep wavepulse which is short in the time required for reaching the maximumvoltage and the time required for returning to the initial voltage isapplied to a ceramic part whereby the energy can be concentrated ontothe portion of the weakest barrier produced by the oxidized layer formedon the surface and inside of the electrode thereby to break through apart of the barrier and reduce the resistance of the barrier.Accordingly, only the barrier formed at the interface of electrodes canbe locally broken and impedance of the ceramic part can be reducedwithout causing deterioration of the capacitor characteristics of theceramic part and without affecting the varistor characteristics.

Furthermore, as the impedance decreases the attenuation rate of noisecan be increased and high frequency noise intruding into signal linescan be effectively absorbed and restrained.

We claim:
 1. A method for manufacturing a ceramic part, which comprisesthe first step of forming a ceramic element of a given shape from asemiconductor ceramic material, the second step of forming at least twoelectrodes on the surface of said ceramic element, the third step offiring the ceramic element having the electrodes, and the fourth step ofapplying at least once between the electrodes of the fired ceramicelement a steep wave pulse voltage of 50 kV or less in maximum valuewhich requires a time of 80 nanoseconds or less for reaching the maximumvalue from an initial value and a time of 1 microsecond or less forreturning to the initial value through the maximum value and which hasan energy of 0.5 joule or lower, thereby breaking through part of abarrier occurring from an oxide layer formed on the surface and insideof the electrodes.
 2. A method according to claim 1 wherein the ceramicmaterial contains as a main component one of SrTiO₃, CaTiO₃, BaTiO₃,ZnO, TiO₂ and MgTiO₃.
 3. A method according to claim 2 wherein theelectrodes contain at least one of Au, Pt, Rh, Pd, Ni, Cr, Zn and Cu. 4.A method according to claim 3 wherein the ceramic element aftersubjected to the first step is calcined in a neutral or oxidizingatmosphere and in the third step the ceramic element having theelectrodes is fired in a reducing atmosphere and then fired again in aneutral or oxidizing atmosphere.
 5. A method for manufacturing a ceramicpart which comprises the first step of forming a sheet from a mixture ofa semiconductor ceramic material, an organic binder and a solvent, thesecond step of stacking a plurality of the sheets to form a laminatehaving a plurality of inner electrodes between the stacked sheets, thethird step of forming on the outer surface of the laminate at least twoouter electrodes electrically connected to the different innerelectrodes, the fourth step of firing the laminate having the outerelectrodes, and the fifth step of applying at least once between theouter electrodes of the fired ceramic laminate a steep wave pulsevoltage of 50 kV or less in maximum value which requires a time of 80nanoseconds or less for reaching the maximum value from an initial valueand a time of 1 microsecond or less for returning to the initial valuethrough the maximum value and which has an energy of 0.5 joule or lower,thereby breaking through part of a barrier occurring from an oxide layerformed on the surface and inside of the electrodes.
 6. A methodaccording to claim 5 wherein the ceramic material contains as a maincomponent one of SrTiO₃, CaTiO₃, BaTiO₃, ZnO, TiO₂ and MgTiO₃.
 7. Amethod according to claim 6 wherein the electrodes contain at least oneof Au, Pt, Rh, Pd, Ni, Cr, Zn and Cu.
 8. A method according to claim 7wherein the laminated after subjected to the first step is calcined in aneutral or oxidizing atmosphere and in the fourth step the ceramicelement having the electrodes is fired in a reducing atmosphere and thenfired again in a neutral or oxidizing atmosphere.
 9. A method formanufacturing a ceramic part, which comprises the first step of forminga sheet from a mixture of a semiconductor ceramic material, an organicbinder and a solvent, the second step of stacking a plurality of thesheets to form a laminate having a plurality of inner electrodes betweenthe stacked sheets, the third step of forming on the outer surface ofthe laminate at least two outer electrodes electrically connected to thedifferent inner electrodes as a first layer, the fourth step of firingthe laminate having the outer electrodes as a first layer in a reducingatmosphere, the fifth step of forming an outer electrode as a secondlayer on the outer electrode formed as the first layer and then firingthe laminate having the first and second outer electrodes in a neutralor oxidizing atmosphere, and the sixth step of applying at least oncebetween the second outer electrodes after fired in the neutral oroxidizing atmosphere a steep wave pulse voltage of 50 kV or less inmaximum value which requires a time of 80 nanoseconds or less forreaching the maximum value from an initial value and a time of 1microsecond or less for returning to the initial value through themaximum value and which has an energy of 0.5 joule or lower, therebybreaking through part of a barrier occurring from an oxide layer formedon the surface and inside of the electrodes.
 10. A method according toclaim 9 wherein the ceramic material contains as a main component one ofSrTiO₃, CaTiO₃, BaTiO₃, ZnO, TiO₂ and MgTiO₃.
 11. A method according toclaim 10 wherein the inner electrodes contain at least one of Au, Pt,Rh, Pd, Ni, Cr, Zn and Cu.